1. Field of the Invention
The present invention relates to methods and apparatus for tuning the wavelength of laser diodes with an external cavity. An ultra-narrow band volume holographic grating is the filtering element of the cavity. The body of the present invention specifically relates to tunable lasers that are self-aligned.
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2. Background Art
It is known that an optical cavity selects one or more wavelengths emitted by a laser amplifier medium. The well known Fabry-Perot cavity comprises two mirrors, one of which is partially transparent. The Fabry-Perot cavity is used for low power laser diodes (tens of mW) as well as high power (tens of Watts) laser diodes. Fabry-Perot cavities resonates at discrete wavelengths. These discrete wavelengths are obtained when an integer number of wavelengths equal one cavity round trip. Because the discrete wavelengths are closely spaced, multiple wavelengths are likely to be amplified by the wide spectrum amplifier medium. The resulting laser is called multimode longitudinal laser.
For certain applications, either a single mode longitudinal laser is required or a reduction in the number of longitudinal modes is beneficial especially for high power multimode lateral laser diodes. It is then necessary to implement a resonant external cavity which includes an additional mean to select the wavelength or group of wavelengths within the Fabry-Perot cavity.
There are numerous external cavity semi-conductor lasers (ECL) apparatus that have been presented in the scientific literature and which have found commercial success over the years. A review of the different architectures for ECL can be found in the “Tunable Lasers Handbook—tunable external-cavity semi-conductor lasers” by P. Zorabedian, chapter 8, Academic Press 1995. The background art for the present invention is related to tunable laser architectures that are self-aligned:
U.S. Pat. No. 5,594,744 discloses an external cavity semi-conductor laser with a self-aligned cavity. FIG. 1 illustrates the prior art. Laser diode 100 is collimated by lens 120. A dispersive grating 140 intercepts the collimated beam and disperse the wavelength angularly into mainly the first order. A retro-reflector prism 130 oriented in a direction orthogonal to the dispersion plane reflects the first order dispersed beam. Because the reflected beam by the retro-reflector 130 has the same direction as the incident beam, the system formed by the dispersive grating and the retro-reflector form a self-aligned filter that send the filtered wavelength back into the laser diode 100.
U.S. Pat. No. 4,942,583 discloses an external cavity semi-conductor laser with a self-aligned cavity. FIG. 2 illustrates the prior art. Laser diode 200 is collimated by lens 220. An interference filter 240 is placed in the path of the collimated beam. By tuning the angle of incidence of the interference filter, the center wavelength can be changed. Lens 260 focuses the collimated beam onto mirror 280 which reflects the light path back into the laser diode 200. This architecture is insensitive to angular tilt and lateral displacement. This laser cavity of this architecture is called degenerate as they are multiple sets of angles and laser spatial location for the laser cavity to be stable.
U.S. Pat. Nos. 5,691,989 and 7,298,771 disclose the use of volume holographic gratings (VHG) as spectrally selective reflectors as one of the mirrors in the external cavity. The volume holographic gratings described in the patents above are not dispersive and not to be confused with the dispersive gratings disclosed in the prior art of U.S. Pat. No. 5,594,744. VHGs filter light by using the Bragg effect. VHGs are well known optical elements described for example in Kogelnick, “Coupled wave theory for thick hologram gratings,” Bell Syst. Tech. J. 48, 2909-2947 (1969). FIG. 3 illustrates the prior art architecture of a tunable laser employing a VHG. laser diode 300 is collimated by lens 320. A reflective (or transmissive) VHG 340 is positioned at an angle in the path of the collimated beam. The diffracted (filtered) beam is directed toward a mirror 360 forming a right angle with the VHG. Mirror 380 retro-reflect the beam path back into the diode 300. By varying the angle of VHG 340 the diffracted (filtered) wavelength is changed accordingly. The tunable external cavity is sensitive to angle changes of mirror 380. Only one specific angle of mirror 380, oriented such that the normal of the mirror is exactly parallel to the incoming beam, will feedback light into the diode 300.